Characteristics of methane emissions from alpine thermokarst lakes on the Tibetan Plateau

Understanding methane (CH4) emission from thermokarst lakes is crucial for predicting the impacts of abrupt thaw on the permafrost carbon-climate feedback. However, observational evidence, especially from high-altitude permafrost regions, is still scarce. Here, by combining field surveys, radio- and stable-carbon isotopic analyses, and metagenomic sequencing, we present multiple characteristics of CH4 emissions from 120 thermokarst lakes in 30 clusters along a 1100 km transect on the Tibetan Plateau. We find that thermokarst lakes have high CH4 emissions during the ice-free period (13.4 ± 1.5 mmol m−2 d−1; mean ± standard error) across this alpine permafrost region. Ebullition constitutes 84% of CH4 emissions, which are fueled primarily by young carbon decomposition through the hydrogenotrophic pathway. The relative abundances of methanogenic genes correspond to the observed CH4 fluxes. Overall, multiple parameters obtained in this study provide benchmarks for better predicting the strength of permafrost carbon-climate feedback in high-altitude permafrost regions.


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This study provided a comprehensive understanding on the spatial patterns, sources and microbial characteristics of CH4 emissions from 120 thermokarst lakes in 30 clusters along a 1,100 km transect on the Tibetan Plateau.
30 representative sampling clusters of thermokarst lakes were selected along a 1,100 km transect on the Tibetan Plateau. The 30 clusters of thermokarst lakes were evenly located in the representative permafrost regions, and located in the alpine grasslands as they represent~80% thermokarst lakes. Gas, water and sediment samples were obtained from thermokarst lakes at each cluster.
Sampling strategy was adopted to maximize the times and number of flux measurements of thermokarst lakes so as to well characterize greenhouse gas emissions. Specifically, at each cluster, four lakes were selected to consider the spatial variability of carbon fluxes among thermokarst lakes. At each lake, 4 to 6 sampling locations was determined by area of lakes, instrumentation performance and field activities which limited the number of samples collected in our study. Each lake was sampled five times at monthly intervals during the ice-free period due to the time limitations. Carbon flux during the ice-free period were only measured due to the harsh field conditions during the ice period.
In-situ total carbon fluxes were determined using an opaque lightweight floating chamber equipped with a closed loop to a nearinfrared laser CH4/CO2 analyzer (GLA231-GGA, ABB., Canada). Specifically, the floating chamber was flushed with ambient air for~10 sec before each measurement. CH4 and CO2 concentrations in the chamber were continuously recorded at an interval of 1 sec after an equilibration period. CH4 and CO2 fluxes were determined as the slope of the linear relationship between their concentrations and measurement time. In addition, CH4 and CO2 concentrations were recorded with the CH4/CO2 analyzer (GLA231-GGA, ABB., Canada). Wind speed and atmospheric temperature with a portable anemometer (Testo 480, Testo SE & Co. KGaA, Lenzkirch, Germany). Air pressure, water temperature, oxidation-reduction potentiality, pH and dissolved oxygen concentration were measured with a portable multiparameter water quality instrument (ProSolo Digital Water Quality Meter, Yellow Springs Instrument, Brannum Lane, USA). Paper, pen, and computers were used for these recoding. The detailed data collection procedure was described in the Methods section. Guibiao Yang, Zhihu Zheng, Luyao Kang and Shuqi Qin were present during data collection.
Carbon fluxes were measured once a month during the open water season (from mid-May to mid-October, 2021) to explore seasonal variation. Due to the high cost of laboratory analyses (such as radio-carbon isotopic analyses and metagenomic sequencing), gas and sediment samples were collected once during mid-July to mid-August, 2021. Sampling was performed across a regional scale (Latitude: 31-39oN, longitude: 91-101oE). Detailed location is available in the supplementary Table 1.
No data were excluded from the analyses.
Replicate measurements were taken for carbon fluxes (five times at monthly intervals during the ice-free period). Reproducibility of experimental design is not relevant, per se, because the ambient conditions are always changing and measurements will be different despite of the same sampling lake.
We collected 120 thermokarst lakes at 30 clusters evenly located in three representative permafrost regions on the Tibetan Plateau. Each thermokarst lake was sampled five times during the ice-free period from mid-May to mid-October. At each cluster, the sampling was taken at multiple locations within multiple lakes, and the gas and sediment samplings were randomly collected and well-mixed in each lake. This strategy increases the area of the sampling and the randomization factors of any sample, which improves the reliability of the generated dataset.